In wireless power delivery systems, composed of a transmitter and one or more receivers (hereafter referred to as the “system”) that rely on an electrical inductor (hereafter referred to as the “transmit coil”) to generate an electromagnetic field that couples to a secondary inductor (hereafter referred to as the “receive coil”). The addition or subtraction of metallic items such as wires, screws, paper clips, or other material that may conduct electrical charge in the presence of an electromagnetic field (hereafter referred to as “parasitic items”) will change the system prior to the addition or subtraction of these parasitic items (hereafter referred to as the “environment”) more specifically the electrical impedance of the transmit coil and/or receive coil. This change will negatively affect some, or all of the following: the ability to deliver power wirelessly to the receiver, the efficiency at which power is delivered to the receiver, other electrical or wireless systems due to interference, and the positional freedom of wireless power delivery (hereafter referred to as “performance”). One method to minimize the effects of parasitic items on performance is to incorporate a magnetic shield, such as a sheet of ferrite and/or metallic material, underneath the transmit coil and/or above the receive coil. This prevents degradation of performance when parasitic items are placed above the receive coil and/or below the transmit coil.
When designing a system with two transmit coils, where one coil is used to generate an electromagnetic field with a fundamental frequency at least one order of magnitude higher or lower than the fundamental frequency for which the coil has been designed, magnetic shielding that can function over both sets of frequencies is desired. In addition, the magnetic shielding must provide adequate shielding for both transmit coils. This is limiting in that the wide range of frequencies may offer few choices of material for effective magnetic shielding, and the magnetic shielding may affect electrical impedance of a receive coil that, in turn, degrades performance of the system.
The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.